It seems to me you should have got a reasonable improvement moving to 8" aperture. The ALPY should accept light at f4 and the diffraction spot shouldn't be limiting with a 10um slit. So its reasonable to expect 75% more light and thus 75% more SNR which means you should be able to get the same SNR in a third of the exposure time (other things being equal). That's nice to have but I guess its only 0.6 in magnitude terms
Actually to a first approximation (ie ignoring sources of noise other than photon noise) SNR is proportional to sqrt(signal) so 1.75xsignal will increase SNR by about 32%
Not sure where your 1/3 exposure time comes from. For the same signal (ignoring the effect of the central obstruction), the exposure time would be (6/8)^2 = 0.56
All this assumes that the star image size (a factor of both psf of the telescope optics and the seeing) is smaller than the slit (10um is very tight and needs excellent seeing)
Edited by robin_astro, 27 March 2019 - 06:52 PM.